The coherent demodulation of digital signals, transmitted by phase modulation and/or amplitude modulation of a carrier, required the generation, on reception, of a reference signal that is phase and frequency coherent with said carrier.
The most widely used amplitude and phase modulation systems do not allow an immediate extraction of said phase and frequency reference signal, because the transmitted signals do not generally contain any spectral component directly correlated with the carrier.
The known systems allowing the extraction of the correct phase reference for such signals are generally based on three different operations.
A first mode of operation consists in multiplying the received-signal frequency by a factor N such that all the products N..DELTA.i (where .DELTA.i are all the possible phase differences in degrees among the various states the modulated carrier can assume) are multiples of 360.degree.. This technique cannot be practically applied when the carrier of the signal to be modulated has a very high frequency and when differences .DELTA.i exist which require a high multiplication coefficient N.
A second mode of operation which can recover the received-signal carrier consists in the received-signal remodulation by a suitable sequence of digital signals, obtained from the same received signal, so as to annul the modulating component of the received signal. This operation needs complex circuits requiring a modulator, delay lines and logic circuits for the generation of the remodulation signal. In addition the errors in transmitted-signal estimate, depending on which the sequence of signals for the remodulation is generated, generate remodulation errors, thus degrading the system performance.
Finally, there is a mode of operation which is based on carrying out linear and non-linear operations on the received-signal baseband components. These components are obtained by filtering the result of the multiplications of the received signal by two references in phase-quadrature with each other.
The phase and frequency of these references under steady condition will be coherent with the frequency and phase of the received-signal carrier.
This method requires very complex circuits when the modulation used has a high number of states.
In these cases to reduce the circuit complexity, simplifications are generally used which degrade the system performance.
In addition, a number of these techniques cannot be efficiently used when the signalling frequency is very high.
All the above-listed methods are, moreover, sensitive to so-called "pattern" noise, due to the well-known intersymbol interference which generally is very high. It is known that in the case of digital signals the signal-to-noise ratio and the intersymbol interference are optimized only at the decision instants at which the transmitted digital-symbol is estimated.